1. Field of the Invention
The present invention relates to methods of treating subterranean zones in formations penetrated by well bores with viscous cross-linked aqueous treating fluids that comprise water, a xanthan biopolymer gelling agent and a combination cross-linker and delayed breaker.
2. Description of the Prior Art
Viscous aqueous treating fluids are used in a variety of operations and treatments in oil and gas wells. Such operations and treatments include, but are not limited to, well completion operations, production stimulation treatments formation permeability conformance control treatments and the like.
An example of a well completion operation involving the use of high viscosity aqueous treating fluids is gravel packing. In gravel packing operations, solid particles referred to in the art as gravel, are carried to a subterranean zone in which a gravel pack is to be placed by a viscous cross-linked aqueous treating fluid. That is, particulate gravel material such as sand is suspended in the viscous cross-linked aqueous treating fluid at the surface and carried to the subterranean zone in which a gravel pack is to be placed. Once the particulate gravel material utilized is placed in the zone, the viscous cross-linked aqueous treating fluid, also referred to as a carrier fluid, breaks into a low viscosity fluid. The gravel pack produced functions as a filter to separate formation solids from produced fluids while permitting the produced fluids to flow into and through the well bore.
An example of a production stimulation treatment utilizing a viscous cross-linked aqueous treating fluid is hydraulic fracturing. That is, a viscous cross-linked aqueous treating fluid referred to in the art as a fracturing fluid, is pumped through the well bore into a subterranean zone to be stimulated at a rate and pressure such that fractures are formed and extended into the subterranean zone. At least a portion of the fracturing fluid carries solid particles such as sand, referred to in the art as proppant, which is carried into the formed fractures. The particulate proppant material is suspended in the viscous cross-linked aqueous fracturing fluid so that the particulate proppant material is deposited in the fractures when the fracturing fluid is broken, i.e., reduced to a low viscosity fluid. The particulate proppant material functions to prevent the formed fractures from closing whereby conductive channels are formed through which produced fluids can flow to the well bore.
The viscous cross-linked aqueous treating fluids utilized heretofore have been viscosified by combining a gelling agent with water. In order to increase the viscosity of the gelled water so that the treating fluid formed is capable of carrying particulate gravel material or particulate proppant material, the gelling agent in the water has been cross-linked by combining a cross-linking agent with the gelled water. In order to break the viscous cross-linked aqueous treating fluid after it has been placed in a subterranean zone, a separate delayed breaker has also been combined with the viscous cross-linked aqueous treating fluid. A particularly suitable gelling agent which is often utilized in gravel packing or fracturing subterranean zones is xanthan biopolymer. In order to cross-link the xanthan biopolymer gelling agent in water, a cross-linking agent such as sodium borate has been added to the gelled water and in order to break the viscous cross-linked aqueous treating fluid formed, a separate delayed breaker such as sodium persulfate has been added to the viscous cross-linked aqueous treating fluid. While the use of separate cross-linking agents and delayed breakers with an aqueous treating fluid containing xanthan biopolymer gelling agent has achieved good results, the cross-linker and delayed breaker have been expensive and require the use of separate pumping and metering equipment. Thus, there is a continuing need for improved cross-linking agents and delayed breakers which are less expensive and require the use of less equipment, etc.